This invention relates to four-stroke spark-ignition engines in which internal combustion of a barrel stratified charge improves fuel efficiency.
Prior to about 1985, most passenger car engines employed one or another combustion chamber configuration with only one intake valve per cylinder. Pentroof chamber configurations with two poppet valves for inducting the engine combustion air (and most often the fuel also) into each engine cylinder have now largely superseded the combustion chamber designs with only a single intake valve. Although the classic four-valve pentroof chamber was incorporated in car racing engines as early as 1912, the unusual placement of the valves in Bristol radial aircraft engines suggests that early designers may not have been very cognizant of the motion of the bulk of the air-fuel charge filling the cylinder at the end of the intake stroke. The combustion chamber configuration of these radial engines (in separate models named Jupiter, Mercury and Pegasus) placed the two intake valves directly opposite each other on facing sides of the pentroof.
In more detail of bulk charge motion, the usual placement of both intake valves on the same side of the pentroof can be combined with appropriate design of the intake passages leading up to these two valves to thus generate a strong swirling motion about an axis perpendicular to the geometric axis of the cylinder. In a longitudinally mounted aircraft engine like the vee-twelve engines widely used in the second World War, this swirl axis parallels the axis about which the aircraft would execute a barrel roll. Barrel swirl is in fact one of the names used to describe the swirl inherent to the classic four-valve pentroof combustion chamber, but tumble is the more commonly used name (unless the engine in question is a barrel stratified charge engine). Even though it is relatively uncommon design practice, barrel swirl can be induced in single intake valve combustion chambers, as demonstrated by Laszlo Hideg in some of the combustion systems disclosed in his U.S. Pat. No. 3,318,292.
This Hideg ""292 patent does include perhaps the earliest disclosure of one type of charge stratification that can easily be induced in a reciprocating engine characterized by barrel swirl. Nevertheless, distinctly segregated barrel swirl layers of fueled and unfueled mixture can more conveniently be generated simply by utilizing two intake valves per engine cylinder so that fuel can be metered into the combustion air inducted through only one of the two intake valves. Just such an arrangement for generating two-layer barrel stratification is disclosed by Mitsubishi engineers Ishida et al. in U.S. Pat. No. 5,050,557. In their U.S. Pat. No. 4,494,504, Honda engineers Yagi et al. also disclose the two-layer type of barrel stratification, including a three-valve combustion chamber of the basic type later employed by Mitsubishi in their first mass produced barrel stratified charge engine as described in SAE paper 920670. Since this particular approach positions the single spark plug in a location generally opposite the one of the intake valves which inducts fueled intake mixture, the single exhaust valve is at least moderately offset from a diametral line of symmetry and thus is doubly compromised in comparison to the flow capacity afforded by the twin exhaust valves of the classic four-valve configuration.
In FIGS. 11B and 12A of Ishida ""557, the Mitsubishi inventors implicitly acknowledge that the original, centrally located spark plug is by itself insufficient when the classic pentroof combustion chamber, with its twin exhaust as well as twin intake valves, functions in a barrel stratified charge operating mode via restriction of fuel delivery to an intake passage serving just one of the two intake valves. This conclusion seems obvious in view of the fact that the central spark plug will lie on the original plane of symmetry, which now theoretically separates the fueled and unfueled barrel swirl layers. As a result, these FIGS. 11B and 12A of Ishida ""557 show an additional spark plug offset nearly all the way to the cylinder wall on the side of the combustion chamber which is fueled during barrel stratified engine operation.
FIG. 12A of Ishida ""557 additionally shows a separate fuel injector located in each of the two intake passages serving the four-valve combustion chamber. This configuration with independent fueling of the two intake passages, augmented by central plus offset ignition, in reality composes the basic structural arrangement for a combustion system according to the present invention. However, FIGS. 12B and 12C of Ishida ""557 proceed with the Mitsubishi inventors"" control strategy and thus verify that their disclosure teaches away from the present invention with its staggered spark timing schedules.
More specifically, FIG. 128 of Ishida ""557 clearly reveals the central spark plug as being inoperative whenever just the one of the two barrel swirl layers enveloping the electrodes of the offset spark plug is fueled. The offset spark plug will provide consistent and reliable ignition at this time, but the much faster burning rate for the ten to ninety percent mass fraction as achieved with central ignition is of course sacrificed. FIG. 12C confirms that only the central spark plug is to be fired when both fuel injectors are activated for nominally homogeneous charge engine operation, to thereby effectively duplicate performance long available from the classic four-valve combustion chamber.
Nevertheless, the full extent to which Ishida ""557 teaches away from the present invention does not become apparent until its drawing FIGS. 13A, 13B, 13C and 14 are considered. These drawing Figures show, first, that engine operation in the barrel stratified charge mode is not altered when the central spark plug is moved to an offset location symmetric to that of the original offset plug. This is true because the second spark plug, now being completely within the unfueled barrel swirl layer, still is not fired during stratified charge engine operation. Therefore, this change in spark plug location does nothing to remedy the slow burn rate experienced in the stratified charge mode. During high BMEP (brake mean effective pressure) engine operation, however, both spark plugs simultaneously ignite the the air-fuel charge which is nominally a homogeneous charge due to the activation of the individual fuel injectors in both intake passages serving the combustion chamber. In discussion specifically of their drawing FIG. 14, Ishida et al. argue that the two offset spark plug locations provide better engine performance in homogeneous charge mode than does the central-plus-offset placement of the two plug locations. This discussion includes neither the possibility of utilizing earlier ignition at the offset location in order to create more favorable conditions for ignition at the central location during stratified charge engine operation, nor the possibility of using the offset spark plug to also improve engine performance during higher BMEP engine operating conditions bordering on homogeneous charge operation.
In U.S. Pat. No. 5,379,743, Ricardo engineers Stokes et al. disclose their own version of what is in effect a barrel stratified combustion system derived from the classic four-valve pentroof combustion chamber by (1) restricting fuel delivery to the combustion air inducted through only one of the two intake valves and (2) augmenting the original centrally located spark plug with another spark plug offset nearly to the cylinder wall on the fueled side of the combustion chamber. Here again, the engineers specify that one or the other, but not both, of the two spark plugs in each combustion chamber be activated in order to accommodate various engine operating conditions. This stipulation may have added significance in the case of the Ricardo engineers because rather extensive development work was based on the Stokes ""743 combustion system, as described in SAE papers 940482 and 950165. Unlike both the present invention and Ishida ""557, Ricardo Consulting Engineers dispensed with the fuel metering system capability for enhancing engine operation at high BMEP levels by delivering fuel to combustion air flowing through both intake valves. Consequently, they were forced to improve air utilization at high BMEP by intentionally degrading separation of the two barrel swirl layers via a short and long combination of intake passage lengths.
As yet another example of modification of the classic four-valve pentroof combustion chamber to render a barrel stratified combustion system, Suzuki engineer Hideharu Oda discloses in U.S. Pat. No. 5,237,973 a completely symmetric combustion chamber configuration featuring two offset spark plugs and independent fueling of air inducted through the two intake valves. Although Ishida ""557 compares an embodiment with these main features to an otherwise identical embodiment with one central and one offset spark plug, as discussed earlier, Oda ""973 does not even mention the central spark plug location.
Returning now to the barrel-stratified combustion systems that have actually reached mass production, Mitsubishi was apparently less than satisfied with their three-valve combustion chamber configuration. Already noted as disadvantages of this configuration are the limited flow capacity of the single exhaust valve and the slow burn rate which results from the single, offset spark plug location. In U.S. Pat. No. 5,295,464, Mitsubishi engineers Ando et al. do specifically mention the restricted flow capacity of the single exhaust valve before embarking on a disclosure of their adaptation of the classic four-valve pentroof combustion chamber for use in a second barrel-stratified passenger car engine that was mass produced. This adaptation abandons two-layer stratification in favor of a central, fueled barrel-swirl layer flanked on each side by an unfueled barrel-swirl layer. Such three-layer barrel stratification of course matches the central spark plug location of the classic four-valve pentroof chamber.
As disclosed in Ando ""464, Mitsubishi""s production four-valve combustion chamber achieves three-layer barrel stratification by using a partition upstream of the valve stem to divide each of the two intake passages into an air flow portion located on the inboard side of the valve stem and another air flow portion on the outboard side. Fuel is injected into the combustion air flowing through the adjacent, inboard air flow portions of each engine cylinder in an upstream location where these two inboard air flow portions are siamesed but still separate from the outboard portions. Fuel is never injected into air flowing through the outboard air flow portions. As an alternative way of realizing this three-layer type of barrel stratification, Mitsubishi""s U.S. Pat. No. 5,237,974 describes a five-valve pentroof combustion chamber in which the combustion air flowing through only the center one of three intake passages is fueled so that the flanking intake passages and their associated intake valves deliver only air. The shortcoming of three-layer barrel stratification is not, however, even mentioned in either of Mitsubishi""s ""464 and ""974 patents, but rather in their SAE paper number 940986.
FIG. 14(b) of this SAE paper shows the two symmetric vortices into which the entire bulk flow of barrel swirl layers is divided as a flat-top piston advances toward its TDC position in a pentroof chamber. By the time the piston has reached a position 15 degrees before TDC, these two vortices have entirely separate swirl rotation axes nearly parallel to the geometric axis of the cylinder itself. In other words, the height of the cylinder volume has been reduced so much by 15 degrees before TDC that the original bulk cylinder flow has been completely broken apart into two symmetric portions each of which has its swirl axis displaced through an angle of almost 90 degrees from the original axis of barrel swirl to thus be almost parallel to the cylinder axis; like conventional axial swirl, these two vortices can survive a large amount of compression by the piston. Most importantly, each of these two vortices now has very strong velocity components traversing the original stratification boundaries of the three-layer type of barrel stratification, but not traversing the single stratification boundary of the two-layer type because the two-layer boundary coincides with the boundary of flow symmetry governing the compression stroke as well as the intake stroke. As a result, two-layer stratification is preserved until quite late in the compression stroke, but three-layer stratification is often degraded quite severely before flame propagation is initiated. While the Mitsubishi engineers say that their three-layer type of barrel stratification is sufficiently preserved at its representative spark timing of 45 degrees before TDC, they also admit that the corresponding piston position for initiation of flame propagation is 15 degrees before TDC. The strong mixing of the fueled and unfueled barrel swirl layers which will likely occur by this time in turn implies that slow combustion will characterize this approach, thus largely nullifying the potential advantage of the central spark plug location in comparison to the offset location of Mitsubishi""s earlier two-layer stratified engine. The overall benefit to three-layer barrel stratification as afforded by Mitsubishi""s xe2x80x9ctumble control pistonxe2x80x9d can be debated, but a representative spark timing advance of 45 degrees certainly suggests slow combustion.
In summary of the technical literature as outlined above, the basic concept of barrel stratification has yet to be tested in the sense that it has always been saddled with at least one serious compromise or another. Stated from a positive perspective, a combustion system representing the true potential of barrel stratification would incorporate the basic elements of (1) the two-layer type of barrel stratification with features which promote and preserve the stratification, (2) independent control of the air-fuel ratio in the barrel swirl layer which is unfueled during fully stratified charge engine operation and (3) a representative crankangle duration on the order of twenty degrees or even less for the ten to ninety percent mass burned fraction during fully stratified engine operation.
The second one of the three basic elements just enumerated in effect requires a fuel metering system which can implement engine operation in a homogeneous charge mode as well as a barrel stratified mode. In his U.S. Pat. No 4,726,343, Herrmann Kruger discloses an engine which utilizes a combustion chamber with two intake valves and a port type of fuel injector in the smaller one of two intake passages serving each cylinder. A single additional fuel injector delivers fuel to the air inducted through all of the larger intake passages because this central type of injector is located in a separate plenum which serves only the larger intake passages; each of the two plenums has its own throttle valve. If Kruger""s engine had a spark plug location appropriate for barrel stratified operation, it could indeed operate in such a mode by maintaining the central fuel injector in a dormant state while appropriately coordinating the positions of the two throttle valves to maintain the same absolute pressure in both plenums. Nevertheless, the central fuel injector could never deliver fuel to combustion air in the other plenum, thus limiting the charge cooling effect of that injector. Furthermore, Kruger ""343 discloses that the central fuel injector is to be dormant only when its associated throttle valve is closed, thus precluding barrel stratified engine operation.
In light of the prior art related to internal combustion engines, it is a primary objective of the present invention to provide a barrel stratified charge engine uncompromised by serious operational defects.
It is another objective to provide at least moderately fast combustion of the ten to ninety percent mass fraction in a two-layer barrel stratified combustion chamber containing a stoichiometrically fueled barrel swirl layer and an unfueled barrel swirl layer.
It is another objective of the present invention to adapt the classic four-valve combustion chamber for barrel stratified engine operation while retaining both consistent ignition at a centrally located spark plug and rapid flame propagation through a barrel swirl layer substantially undiluted by mixing with an unfueled barrel swirl layer.
It is yet another objective to employ combustion of a premixed stratified charge to improve the fuel efficiency of a spark ignition engine, but without the complexity of a divided combustion chamber.
It is still another objective to present a highly efficient spark-ignition engine which burns vaporized fuel rather than the fuel droplets of a fuel aerosol generated by injection of the fuel directly into the combustion chamber.
It is further an objective to present a barrel stratified combustion system which accommodates a wide BMEP range of unthrottled engine operation.
It is another objective of the present invention to present a barrel stratified engine which combines the advantages of port fuel injection at low to medium BMEP levels with the full charge cooling benefit of central fuel injection at maximum levels of BMEP.
These and other objectives can be achieved, in the case of a multi-cylinder engine, by employing a premixed-charge combustion system which utilizes the two-layer type of barrel stratification as well as, in a specifically preferred embodiment, four poppet valves arranged symmetrically with respect to the plane which separates the two barrel swirl layers. A first spark plug occupies an offset location positioned well into the always-fueled barrel swirl layer because this offset location is bounded by only the exhaust valve and only the intake valve on the fueled side of the engine cylinder. A second spark plug occupies a central location bounded by all four of the poppet valves, but the strong cycle-to-cycle variation in air-fuel ratio existing at this location prior to combustion in the fueled barrel swirl layer does not translate into strong cyclical variation of BMEP because combustion is initiated first by the offset spark plug. The central spark plug is fired only after expansion of combustion products from ignition at the offset spark plug has pushed the rich-side boundary of the zone of cycle-to-cycle variation in air-fuel ratio past the electrodes of the central spark plug. Consequently, the central spark plug can be consistently fired when its electrodes are enveloped by fresh mixture quite similar in air-fuel ratio to that which exists at the offset spark plug. In addition, the combustion system includes individual port type fuel injectors for fully barrel stratified charge engine operation and a central fuel metering system for homogeneous charge operation with essentially maximum charge cooling effect. In a preferred method of utilizing combined as well as separate fuel delivery from either the individual or the central fuel metering components, the engine operates unthrottled throughout a wide range of higher levels of BMEP.